Although researchers studying adolescent aggression have proposed a conceptual distinction between physical and relational aggression, there is contradictory evidence regarding the degree to which they differ in their trajectories and relations to other outcomes. This study explored the importance of differentiating between these two forms of aggression based on comparisons of their trajectories, relation with each other, impact on delinquency and substance use, and gender differences. Data were collected as part of the Multisite Violence Prevention Project, conducted at 19 middle schools from four sites with a predominantly low-income, minority sample of students (N = 2,822). Growth curves showed significant linear increases and quadratic trends for physical and relational aggression. Boys and girls had similar shaped trajectories, but boys reported significantly higher levels of physical aggression than girls. Bivariate latent growth curve models and autoregressive models suggested that physical aggression was a stronger predictor of externalizing difficulties than relational aggression.